Carbon is known to have five unique crystalline structures, including diamond, fullerene (0-D nano graphitic material), carbon nano-tube (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material).
The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material.
A graphene plane in a graphite crystal is composed of carbon atoms occupying a two-dimensional, hexagonal lattice. The constituent graphene planes of a graphite crystal can be extracted or isolated from a graphite crystal to form individual graphene sheets. An isolated, individual graphene sheet is commonly referred to as single-layer graphene. A stack of multiple graphene planes bonded through van der Waals forces in the thickness direction is commonly referred to as a multi-layer graphene, typically having up to 300 layers or graphene planes (<100 nm in thickness), but more typically up to 30 graphene planes (<10 nm in thickness), even more typically up to 20 graphene planes (<7 nm in thickness), and most typically up to 10 graphene planes (commonly referred to as few-layer graphene in scientific community). Single-layer graphene and multi-layer graphene sheets are collectively called “nano graphene platelets” (NGPs). Our research group pioneered the development of graphene materials and related production processes as early as 2002: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted in October 2012; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006).
NGPs are typically obtained by intercalating natural graphite flakes with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound (GIC) or graphite oxide, as illustrated in FIG. 1. This is most often accomplished by immersing natural graphite flakes in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium chlorate). The resulting GIC is actually some type of graphite oxide particles. This GIC is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion which contains discrete and visually discernible graphite oxide particles dispersed in water. There are two processing routes to follow after this rinsing step:
Route 1 involves removing water from the suspension to obtain “expandable graphite,” which is essentially a mass of dried GIC or dried graphite oxide particles. Upon exposure of expandable graphite to a temperature in the range of typically 800-1,050° C. for approximately 30 seconds to 2 minutes, the GIC expands by a factor of 30-300 to form “graphite worms,” which are each a collection of exfoliated, but largely un-separated or interconnected graphite flakes. In Route 1A, these graphite worms (exfoliated graphite or “networks of interconnected/non-separated graphite flakes”) can be re-compressed to obtain flexible graphite sheets that typically have a thickness in the range of 0.125 mm (125 μm)-0.5 mm (500 μm). One may choose to use a low-intensity air mill or shearing machine to simply break up the graphite worms for the purpose of producing the so-called “expanded graphite,” which contain mostly graphite flakes having a thickness greater than 100 nm (hence, not a nano material by definition).
Exfoliated graphite worms, expanded graphite, and the recompressed mass of graphite worms (commonly referred to as flexible graphite sheet or flexible graphite foil) remain as a 3-D graphitic material that is fundamentally different and patently distinct from either the 1-D nano carbon material (CNT) or the 2-D nano carbon material (graphene).
As disclosed by M. Smalc, et al, U.S. Pat. No. 7,292,441 (Nov. 6, 2007) and U.S. Pat. No. 6,982,874 (Jun. 3, 2006), and J. W. Tzeng, U.S. Pat. No. 6,482,520 (Nov. 19, 2002), these flexible graphite (FG) foils can be used as a heat spreader material, but exhibiting a maximum in-plane thermal conductivity of typically less than 500 W/mK (more typically <300 W/mK) and in-plane electrical conductivity no greater than 1,500 S/cm.
In Route 1B, the exfoliated graphite is subjected to high-intensity mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill) to form separated single-layer and multi-layer graphene sheets (collectively, NGPs), as disclosed in our U.S. application Ser. No. 10/858,814. Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm. In the present application, the thickness of multi-layer NGPs is preferably less than 20 nm.
Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating/isolating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation has been increased from 0.335 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separated, isolated, or discrete graphene oxide (GO) sheets. These graphene oxide sheets can then be chemically or thermally reduced to obtain “reduced graphene oxides” (RGO) typically having an oxygen content of 0.01%-10% by weight, more typically 0.01%-5% by weight.
For the purpose of defining the claims of the instant application, NGPs include single-layer and multi-layer graphene or reduced graphene oxide with an oxygen content of 0-10% by weight, more typically 0-5% by weight, and preferably 0-2% weight. Pristine graphene has essentially 0% oxygen. Graphene oxide (including RGO) can have 0.01%-46% by weight of oxygen. The graphene oxide gel, to be described in detail later, typically contains 20-46% by weight oxygen.
It may be noted that flexible graphite sheets or foils (obtained by re-compressing exfoliated graphite worms) for electronic device thermal management applications (e.g. as a heat spreader) have the following major deficiencies:                (1) As indicated earlier, flexible graphite (FG) foils exhibit a relatively low thermal conductivity, typically <500 W/mK and more typically <300 W/mK.        (2) Flexible graphite foils are also of low strength and poor structural integrity. The high tendency for flexible graphite foils to get torn apart makes them difficult to handle in the process of integrating them in a microelectronic device.        (3) Another very subtle, largely ignored or overlooked, but critically important feature of FG foils is their high tendency to get flaky with graphite flakes easily coming off from FG sheet surfaces and emitting out to other parts of a microelectronic device. These highly electrically conducting flakes (typically 1-500 μm in lateral dimensions and >100 nm in thickness) can cause internal shorting and failure of electronic devices.        (4) For this reason, it is necessary to apply a protective resin coating onto a surface or on both surfaces of a flexible graphite foil in order to prevent graphite flakes from being released. This resin coating is typically not a thermally or electrically conductive material that is often an undesirable feature in a situation where high conductivity is required. In other situations where electrical insulation or isolation is required, this resin layer can present some issues (e.g. mis-match in coefficients of thermal expansion and elastic constants between the FG layer and the resin coating, resulting in delamination or peeling-off after some number of thermal cycles).        
Other sheet-like graphitic structures intended for use as a heat spreader or thermal interface material include carbon nano-tube (CNT) paper (e.g. Bucky paper), carbon fiber mat (e.g. carbon nano-fiber or CNF mat), and carbon paper (e.g. made of short carbon fibers). These graphitic sheets also suffer from similar shortcomings as FG foils. For instance, although individual CNTs or CNFs can exhibit a high thermal conductivity (1,500-3000 W/mK), the resulting CNT or CNF paper or mat typically exhibit an in-plane thermal conductivity less than 100 W/mK and often less than 10 W/mK, likely due to few and poor contacts between individual CNT or CNF filaments, providing insufficient cross-sections for electron flow or impeding electron flow. Further, the contact between a sheet-like graphitic layer and a heat source is usually poor due to limited contact surfaces between such a graphitic layer (e.g. CNT paper) and a rigid device component (e.g. a CPU in a mobile phone). This results in an ineffective heat transfer between the heat source and the graphitic layer.
Similarly, the NGPs, when packed into a film or paper sheet of non-woven aggregates, exhibit a thermal conductivity higher than 1,000 W/mK only when the film or paper is cast and pressed into a sheet having a thickness lower than 10 μm, and higher than 1,500 W/mK only when the film or paper is cast and pressed into a sheet having a thickness lower than 1 μm. This is reported in our earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9, 2007). However, ultra-thin film or paper sheets (<10 μm) are difficult to produce in mass quantities, and difficult to handle when one tries to incorporate these thin films as a heat spreader material during the manufacturing of microelectronic devices. Further, thickness dependence of thermal conductivity (not being able to achieve a high thermal conductivity at a wide range of film thicknesses) is not a desirable feature. Non-woven aggregates of NGPs (graphene sheets or platelets) also have a tendency to get flaky.
Our earlier application (U.S. application Ser. No. 11/784,606) further disclosed a mat, film, or paper of NGPs infiltrated with a metal, glass, ceramic, resin, and CVD graphite matrix material. Later on, Haddon, et al (US Pub. No. 2010/0140792, Jun. 10, 2010) also reported NGP thin film and NGP-polymer composites for thermal management applications. The processes used by Haddon et al to produce NGPs are identical to those disclosed much earlier by us (Jang, et al. US patent application Ser. No. 10/858,814 (Jun. 3, 2004)). The NGP-polymer composites, as an intended thermal interface material, have very low thermal conductivity, typically <<2 W/mK. The NGP films of Haddon, et al are essentially non-woven aggregates of graphene platelets, identical to those of our earlier invention (U.S. application Ser. No. 11/784,606). Again, these aggregates have a great tendency to have graphite particles flaking and separated from the film surface, creating internal shorting problem for the electronic device containing these aggregates. They also exhibit low thermal conductivity unless made into thin films (10 nm-300 nm, as reported by Haddon, et al) which are very difficult to handle in a real device manufacturing environment. Balandin, et at (US Pub. No. 2010/0085713, Apr. 8, 2010) also disclosed a graphene layer produced by CVD deposition or diamond conversion for heat spreader application. More recently, Kim, et al (N. P. Kim and J. P. Huang, “Graphene Nanoplatelet Metal Matrix,” US Pub. No. 2011/0108978, May 10, 2011) reported metal matrix infiltrated NGPs. However, metal matrix material is too heavy and the resulting metal matrix composite does not exhibit a high thermal conductivity.
Another prior art material for thermal management application is the pyrolitic graphite film. The lower portion of FIG. 1 illustrates a typical process for producing prior art pyrolitic graphitic films or sheets. The process begins with carbonizing a polymer 46 at a carbonization temperature of 500-1,000° C. for 2-10 hours to obtain a carbonized material 48, which is followed by a graphitization treatment at 2,500-3,200° C. for 5-24 hours to form a graphitic film 50. This is a slow, tedious, and energy-intensive process. Furthermore, carbonization of certain polymers (e.g. polyacrylonitrile) involves the emission of toxic species.
Thus, it is an object of the present invention to provide a highly thermally conductive thin film laminate composed of a graphitic core or substrate layer (having two primary, opposed surfaces) and a graphene oxide (GO) coating layer deposited on one primary surface of the core/substrate layer, or two graphene oxide coating layers deposited on the two primary surfaces of the core graphitic layer. The graphitic substrate layer can include, but not limited to, flexible graphite foil, graphene film, graphene paper, graphite particle paper, carbon-carbon composite film, carbon nano-fiber paper, or carbon nano-tube paper.
The present invention also provides a method or process for producing such a GO-coated laminate. The graphene oxide (GO) coating layer is initially a layer of GO gel when the gel is first deposited onto a primary surface of a graphitic core/substrate layer. The liquid component of this GO gel is then partially or totally removed and, concurrently or sequentially, this GO coating layer is thermally converted to an integrated graphene film obtained by heat-treating graphene oxide gel to merge individual graphene oxide molecules in an edge-to-edge manner.
This thermally conductive laminate can be used for thermal management applications (e.g. for use as a heat spreader) in a microelectronic device, such as a mobile phone (including a smart phone), a notebook computer, a tablet, an e-book, a telecommunication device, and any hand-held computing device or portable microelectronic device.
It is another object of the present invention to provide a GO-coated graphitic laminate that exhibits a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, surface smoothness, surface hardness, and scratch resistance unmatched by any thin-film material of comparable thickness range.
Still another object of the present invention is to provide a GO-coated 2-layer or 3-layer laminate (or n-layer, n being up to 1000) that exhibits exceptional thermal conductivity, electrical conductivity, strength, and reduced or eliminated tendency to flake-off.
It is yet another object of the present invention to provide a highly conductive GO-coated laminate thin-film sheet that meets the following technical requirements (a) in-plane thermal conductivity greater than 600 W/mK (preferably greater than 1,000 W/mK, and further preferably greater than 1,700 W/mK); (b) in-plane electrical conductivity is greater than 2,000 S/cm (preferably >3,000 S/cm, more preferably >5,000 S/cm, and most desirably >10,000 S/cm); (c) Rockwell surface hardness value >60 (preferably >80); and/or (d) a tensile strength greater than 10 MPa (preferably >40 MPa, more typically >60 MPa, and most preferably >100 MPa).
Another object of the present invention is to provide an electrically and thermally conductive graphitic laminate that contains a unitary graphene oxide layer or graphene single crystal coated on a major surface of a graphitic substrate layer.
It is still another object of the present invention to provide a cost-effective process of producing GO-coated graphitic foil laminates that exhibit a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, surface hardness, and scratch resistance unmatched by any thin-film graphitic material of comparable thickness range.